Apparatus for thermal ablation

ABSTRACT

A catheter for use in an electrophysiological procedure to ablate a site includes a metallic tip having a first work function and energized by a source of RF energy. The RF energy return path is through a relatively large plate of a metallic material having a second work function and disposed at a location removed from the ablation site. The difference in work functions of the tip and the plate, operating in the presence of an electrolyte represented by the intermediate tissue, produces an exchange of electrical charges through chemical reaction to create a galvanic cell. By loading the galvanic cell with a shunt resistor, it becomes a current source providing a current linear with and highly dependent on the tissue temperature at the ablation site. This accurate representation of the tissue temperature at the ablation site is used to regulate the RF energy applied to maintain the tissue at the ablation site at a predetermined temperature during the ablation procedure.

This is a division of application Ser. No. 08/990,877 filed on Dec. 15,1997 now U.S. Pat. No. 6,013,074 E. Taylor for “APPARATUS AND METHOD FORTHERMAL ABLATION”, which is continuation application of Ser. No.08/488,887 filed Jun. 9, 1995 entitled “APPARATUS AND METHOD FOR THERMALABALATION”, now U.S. Pat. No. 5,697,925.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to catheters and, more particularly,temperature controlled catheter probes for ablating tissue.

2. Background of the Invention

The heart is a four chamber muscular organ (myocardium) that pumps bloodthrough various conduits to and from all parts of the body. In orderthat the blood be moved in the cardiovascular system in an orderlymanner, it is necessary that the heart muscles contract and relax in anorderly sequence and that the valves of the system open and close atproper times during the cycle. Specialized conduction pathways conveyelectrical impulses swiftly to the entire cardiac muscle. In response tothe impulses, the muscle contracts first at the top of the heart andfollows thereafter to the bottom of the heart. As contraction begins,oxygen depleted venous blood is squeezed out of the right atrium (one oftwo small upper chambers) and into the larger right ventricle below. Theright ventricle ejects the blood into the pulmonary circulation, whichresupplies oxygen and delivers the blood to the left side of the heart.In parallel with the events on the right side, the heart muscle pumpsnewly oxygenated blood from the left atrium into the left ventricle andfrom there out to the aorta which distributes the blood to every part ofthe body. The signals giving rise to these machinations emanates from acluster of conduction tissue cells collectively known as the sinoatrial(SA) node. The sinoatrial node, located at the top of the atrium,establishes the tempo of the heartbeat. Hence, it is often referred toas the cardiac pacemaker. It sets the tempo simply because it issuesimpulses more frequently than do other cardiac regions. Although thesinoatrial node can respond to signals from outside the heart, itusually becomes active spontaneously. From the sinoatrial node impulsesrace to the atrioventricular (AV) node above the ventricles and speedalong the septum to the bottom of the heart and up along its sides. Theimpulses also migrate from conduction fibers across the overlying musclefrom the endocardium to the epicardium to trigger contractions thatforce blood through the heart and into the arterial circulation. Thespread of electricity through a healthy heart gives rise to the familiarelectrocardiogram. Defective or diseased cells are abnormalelectrically. That is, they may conduct impulses unusually slowly orfire when they would typically be silent. These diseased cells or areasmight perturb smooth signalling by forming a reentrant circuit in themuscle. Such a circuit is a pathway of electrical conduction throughwhich impulses can cycle repeatedly without dying out. The resultingimpulses can provoke sustained ventricular tachycardia: excessivelyrapid pumping by the ventricles. Tachycardia dysrhythmia may imposesubstantial risk to a patient because a diseased heart cannot usuallytolerate rapid rates for extensive periods. Such rapid rates may causehypotension and heart failure. Where there is an underlying cardiacdisease, tachycardia can degenerate into a more serious ventriculardysrhythmia, such as fibrillation. By eliminating a reentrant circuit orsignal pathway contributing to tachycardia, the source of errantelectrical impulses will be eliminated. Ablation of the site attendantsuch a pathway will eliminate the source of errant impulses and theresulting arrhythmia. Mapping techniques for locating each of such sitesthat may be present are well known and are presently used.

Interruption of the errant electrical impulses is generally achieved byablating the appropriate site. Such ablation has been formed by lasers.The most common technique for use at an ablation site involves the useof a probe energized by radio frequency radiation (RF). Regulation andcontrol of the applied RF energy is through a thermistor locatedproximate the RF element at the tip of a catheter probe. While such athermistor may be sufficiently accurate to reflect the temperature ofthe thermistor, it inherently is inaccurate in determining thetemperature of the tissue at the ablation site. This results fromseveral reasons. First, there is a temperature loss across the interfacebetween the ablation site and the surface of the RF tip. Second, theflow of blood about a portion of the RF tip draws off heat from theablation site which must be replenished by increasing the temperature ofthe tissue under ablation. However, temperatures above 100° C. result incoagulum formation on the RF tip, a rapid rise in electrical impedanceof the RF tip, and loss of effective tissue heating. Third, there is alag in thermal conduction between the RF tip and the thermistor, whichlag is a function of materials, distance, and temperature differential.Each of these variables changes constantly during an ablation procedure.To ensure that the ablation site tissue is subjected to heat sufficientto raise its temperature to perform irreversible tissue damage, thepower transmitted to the RF tip must be increased significantly greaterthan that desired for the ablation site in view of the variable losses.Due to the errors of the catheter/thermistor temperature sensingsystems, there is a propensity to overheat the ablation site tissueneedlessly. This creates three potentially injurious conditions. First,the RF tip may become coagulated. Second, tissue at the ablation sitemay “stick to” the RF tip and result in tearing of the tissue uponremoval of the probe. This condition is particularly dangerous when theablation site is on a thin wall of tissue. Third, inadequate tissuetemperature control can result in unnecessary injury to the heartincluding immediate or subsequent perforation.

When radio frequency current is conducted through tissue, as might occurduring a procedure of ablating a tissue site on the interior wall(endocardium) of the heart with a radio frequency energized catheter,heating occurs preliminarily at the interface of the tip of the catheterand the myocardial tissue. Given a fixed power level and geometry of thecatheter probe, the temperature gradient from the probe interface and adistance, r, into the tissue is proportional to 1/r⁴. Heating is causedby the resistive (OHMIC) property of the myocardial tissue and it isdirectly proportional to the current density. As may be expected, thehighest temperature occurs at the ablation site which is at theinterface of the RF tip and the tissue.

When the temperature of the tissue at the ablation site approaches 100°C., a deposit is formed on the RF tip that will restrict the electricalconducting surface of the RF tip. The input impedance to the RF tip willincrease. Were the power level retained constant, the interface currentdensity would increase and eventually arcing and carbonization wouldoccur. At these relatively extreme temperatures, the RF tip will oftenstick to the surface of the tissue and may tear the tissue when the RFtip is removed from the ablation site.

To effect ablation, or render the tissue nonviable, the tissuetemperature must exceed 50° C. If the parameters of the RF tip of acatheter are held constant, the size and depth of the lesion caused byablation is directly proportional to the temperature at the interface(assuming a time constant sufficient for thermal equilibrium). In orderto produce lesions of greatest depth without overheating of the tissuesat the interface, critical temperature measurement techniques of the RFtip are required.

The current technology for measuring the temperature of an RF tipembodies a miniature thermistor(s) located in the RF tip of the probe.The present state of the art provides inadequate compensation for thethermal resistance that exists between the thermistor and the outersurface of the RF tip or between the outer surface of the RF tip and thesurface of the adjacent tissue. Because of these uncertaintiescontributing to a determination of the specific temperature of thetissue at the interface, apparatus for accurately measuring theinterface temperature would be of great advantage in performing anelectrophysiological procedure to ablate a specific site(s) of themyocardial tissue.

SUMMARY OF THE INVENTION

A catheter probe having a tip energized by an RF generator radiates RFenergy as a function of the RF energy applied. When the tip is placedadjacent tissue at an ablation site, the irradiating RF energy heats thetissue due to the ohmically resistive property of the tissue. Thecatheter tip placed adjacent the ablation site on tissue in combinationwith an electrically conducting plate in contact with tissue at alocation remote from the ablation site and an electrolyte defined by theintervening tissue create a galvanic cell when the tip and plate havedifferent work functions because of migration of electrical chargestherebetween. By loading the galvanic cell, the DC output current is alinear function of the temperature of the ablation site heated by the RFenergy. The DC output current of the galvanic cell is used to regulatethe output of the RF generator applied to the catheter tip to controlthe current density at the ablation site and hence maintain the ablationsite tissue at a predetermined temperature.

It is therefore a primary object of the present invention to provideregulation of RF energy radiated from a catheter tip during an ablationprocedure.

Another object of the present invention is to provide an output signalrepresentative of the tissue temperature at an ablation site forregulating the RF radiation power level of a probe performing anablation procedure.

Yet another object of the present invention is to generate a signalrepresentative of the actual tissue temperature at an ablation site tocontrol heating of the ablation site by regulating irradiating RF energyfrom an ablating RF tip despite heat dissipating variables that may bepresent.

Still another object of the present invention is to provide a catheterfor ablating cardiac impulse pathways at a predetermined temperature.

A further object of the present invention is to provide aself-regulating catheter mounted RF radiating element controlled by thetissue temperature at an ablation site for ablating a site on theendocardium of a heart suffering tachycardia dysrhythmia to destroy apathway of errant electrical impulses at least partly contributing tothe tachycardia dysrhythmia.

A still further object of the present invention is to provide a methodfor heating and ablating an ablation site at a predetermined andcontrollable temperature.

These and other objects of the present invention will become apparent tothose skilled in the art as the description thereof proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be described with greater specificity andclarity with reference to the following drawings, in which:

FIG. 1 illustrates a simplified representation of the present invention;

FIG. 2 illustrates the current density at an ablation site during anablation procedure;

FIG. 3 illustrates a representation of a catheter probe embodying athermistor useful in the present invention;

FIG. 4 illustrates representatives curves for calibrating thetemperature of an ablation site through use of a probe embodying athermistor;

FIG. 5 is a block diagram illustrating the circuitry attendant thepresent invention; and

FIG. 6 illustrates a catheter probe for sequentially mapping theendocardium, identifying a site to be ablated and ablating the sitewithout relocating the probe.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Two electrodes of different metals having different work functions inthe presence of an electrolyte, such as blood, a saline solution orliving tissue, will produce an exchange of electrical charges and anelectromotive force (emf) is generated. This emf generator is known as agalvanic cell. A technical discussion of the history of galvanic cellsis set forth in Chapter 1.3, entitled “Basic Electrochemistry” (pages12-31) in a textbook entitled Modern Electrochemistry, authored by JohnO'M. Bockris, published by Plenum Press., New York, dated 1970. Detailedtechnical discussions of galvanic cells can be found in: Chapter 4,entitled “Reversible Electrode Potentials” (pages 73-100) of a textbookentitled Electrochemistry Principles and Applications, authored byEdmund C. Potter, published by Cleaver-Hume Press, Ltd., dated 1956;Chapter 4 entitled “Electrodes and Electrochemical Cells” (pages 59-89)of a textbook entitled Introduction to Electrochemistry, authored by D.Bryan Hibbert, published by MacMillan Press Ltd., dated 1993; andChapter 12 entitled “Reversible Cells” (pages 282-311) of a textbookentitled Electrochemistry of Solutions, authored by S. Glasstone,published by Methuen & Co. Ltd., London, dated 1937 (Second Edition).These technical discussions are incorporated herein by reference.

The magnitude of the potential of a galvanic cell is a function of theelectrolyte concentrates and the metals' work functions. The opencircuit voltage of the galvanic cell is essentially constant despitetemperature changes at the interface between the electrodes and theelectrolyte. However, by loading the galvanic cell with a fixed valueshunt resistance it simulates a current generator which has an outputsignal directly proportional to the temperature of the metal andelectrolyte interface. The output signal of the current generator can becalibrated as a function of the temperature at the interface. A simplemethod for calibration is that of referencing the output of the currentgenerator with the output of a thermistor embedded in the electrode atsteady state power and temperature conditions at an initial or firsttemperature and at a second temperature. This will provide two datapoints for the power/temperature curve of the current generator. Sincethe output of the current generator is linear, the curve can be extendedto include all temperatures of interest.

The present invention is directed to apparatus for ablating an errantcardiac conduction pathway responsible for or contributing to arrhythmiaof the heart. The ablation process is performed by heating the ablationsite tissue to a temperature typically exceeding 50° C., sufficient tocause ablation of the cells contributing to the errant impulse pathway.The ablation is effected by irradiating the ablation site tissue withradio frequency (RF) energy. For this purpose, a catheter probe tip ispositioned adjacent the ablation site, which site has been previouslydetermined by mapping procedures well known to physicians and thoseskilled in the art. Upon positioning of the probe tip at the ablationsite, a source of RF energy is actuated to transmit RF energy through aconductor to the tip of the probe. The RF energy radiates from the tipinto the ablation site tissue. The current density at the ablation siteis a function of the power of the RF energy irradiating the ablationsite and the surface area defining the interface between the tip and theablation site tissue. Control of the tissue temperature at the interfaceis of significant importance to control the area and depth of ablationin order to perform the degree of ablation necessary, to preventcoagulation on the tip, to prevent the tip from sticking to the tissue,to prevent avoidable injury to adjacent tissue, to prevent perforationof the tissue, and to avoid unnecessary heating of the blood flowing inand about the tip.

Catheter probes having a thermistor embedded at the tip have been usedto perform an ablation procedure and the amount of RF energy applied hasbeen regulated as a function of the temperature sensed by thethermistor. Such temperature sensing is inherently inaccurate indetermining the temperature at the ablation site due to the numerousvariables present. First, there exists a temperature loss through theinterface between the ablation site and the surface area of the tip incontact with tissue. Second, there exists a thermal conductivity lagbetween the surface area of the tip in contact with the ablation siteand the thermistor. Third, the orientation of the tip with respect tothe ablation site will vary with a consequent variation of heating ofthe ablation site. Finally, the blood flowing about the tip area not intissue contact will draw off heat as a function of both flow rate andorientation of the tip with respect thereto. By experiment, it has beenlearned that the differences between the tissue temperature at theablation site and the temperature registered by a thermistor may rangefrom 10° C. to 35° C. Such temperature excursion may result inunnecessary injury without a physician being aware of the injury causedat the time of the ablation procedure. Where ablation is being performedupon a thin wall myocardium, puncturing can and does occur withpotentially disastrous results.

The present invention is shown in simplified format in FIG. 1. An RFgenerator 10 serves as a source of RF energy. The output of the RFgenerator is controlled by an input signal identified as J₁. The RFenergy, as controlled by J₁, is transmitted through a conductor 12 to acatheter probe 14. This probe is depicted as being lodged within a bloodfilled chamber 16 of a heart. The chamber may be the right or leftatrium or the right or left ventricle. Probe 14 is lodged adjacent, forinstance, tissue 18 at an ablation site 20 representing a reentrantcircuit to be ablated. As represented, blood continually flows throughchamber 16 about and around probe 14.

Probe 14 includes a tip 30 electrically connected to conductor 12 toirradiate ablation site 20 with RF energy. Typically, the frequency maybe in the 350 kHz to 1200 kHz range. Such irradiation of the ablationsite will result in heating of the ablation site as a function of thecurrent density at the ablation site. The current density is determinedby the energy level of the irradiating RF energy and the surface area ofthe ablation site. More specifically, the heat generated is proportionalto the current density squared. This may be expressed as:T(r)=kPd=kI²R=(J_(O) ²/r⁴) R, where T=temperature, r=distance from theinterface, J₀=current density at the interface, Pd=power dissipated,I=current at the interface, and R=resistance at the interface. Thereturn path to RF generator 10 is represented by conductor 32. Conductor32 is electrically connected to a relatively large sized plate 34 placedadjacent the patient's skin, preferably a large surface area of thepatient's back. To ensure good electrical contact, an electricallyconducting salve may be disposed intermediate plate 34 and patient'sback 36. The fluid and tissues of the patient intermediate tip 30 andplate 34, represented by numeral 38, constitutes, in combination, anelectrolyte and therefore an electrically conductive path between thetip and the plate. The DC current flow is represented by i, and the DCvoltage is represented by V_(s).

As more particularly illustrated in FIG. 2, ablation site 20 has arelatively high concentration of current paths, representativelydepicted by diverging lines identified with numerals 42, 44, 46, 48, 50,and 52. These current paths are in close proximity with one another atthe ablation site. The resulting high current density will produceheating of the ablation site as a function of the current density. Thedepth of the ablated tissue is representatively illustrated by line 54.The current density proximate back 36 of the patient adjacent plate 34is relatively low. With such low current density, essentially no heatingof the skin adjacent plate 34 will occur. It is to be appreciated thatFIG. 2 is not drawn to scale and is intended to be solely representativeof relative current densities resulting from irradiation of an ablationsite by tip 30.

Ablation with tissue temperature control permits the physician tooptimize the ablation process by allowing the ablation to occur atmaximum temperature that is below a temperature conducive to formationof coagulation on the tip. Since such temperature is a function of theRF energy irradiating the ablation site tissue, control of the amount ofRF energy transmitted via conductor 12 to the tip is necessary. Apresently available type of catheter probe 60 is illustrated in FIG. 3.This probe includes a tip 62 for radiating RF energy received throughconductor 64 from a source of RF energy. A thermistor 66 is embedded intip 62 or in sufficient proximity with the tip to be responsive to thetemperature of the tip. A pair of conductors 68 and 70 interconnectthermistor 66 with a signal detection circuit to provide an outputsignal representative of the temperature sensed. Furthermore, probe 60may include mapping electrodes 72, 74 and 76. These electrodes may beused in conjunction with manipulation of probe 60 within the heart todetect and identify errant impulse pathways causing cardiac arrhythmia.Conductors 78, 80 and 82 connect electrodes 72, 74 and 76, respectively,to circuitry associated with the mapping functions, as is well known.

As stated above, thermistor 66 is incapable of providing an accuraterepresentation of the temperature at the ablation site. In summary,these causes are due to heat loss through the interface between tip 30and ablation site 20 (see FIG. 2), thermal lag between the area oftissue in contact with the tip and the sensing element of thethermistor, and heat loss resulting from flow of blood about the tiparea not in contact with the tissue.

By experimentation, it has been learned that the combination of tip 30,plate 34 and body 38 perform in the manner of a galvanic cell providedthat the tip and the plate are metallic and of different work functionssince body 38 acts as an electrolyte; the body is permeated by fluidshaving electrical properties similar to a saline solution. Experimentsindicate that a preferable material for tip 30 is platinum and apreferable material for plate 34 is copper. The open circuit voltage(v_(s)) of this galvanic cell is essentially independent of thetemperature of ablation site 20. However, if the galvanic cell isheavily loaded with a shunt resistor, the galvanic cell serves as acurrent source and the magnitude of the current (i_(s)) is linear as afunction of the tissue temperature at the ablation site through the 37°C. to 100° C. temperature range of interest. The temperature of thetissue adjacent plate 34 is the body temperature since the currentdensity is insufficient to generate heat of any consequence. Thus, thegalvanic cell created by the apparatus illustrated in FIG. 2 provides anoutput signal representative of the tissue temperature at ablation site20 and irrespective of the temperature of tip 30.

One method for calibrating the galvanic cell will be described, butother methods may be used which do not require the presence of athermistor at the tip. A thermistor is embedded in the tip of a catheterprobe, such as probe 60. For reasons set forth above, the output of thethermistor is inherently inaccurate with respect to the actual tissuetemperature at the ablation site; moreover, the temperature sensed bythe thermistor as a function of the power applied is generallynonlinear. However, within a temperature range from a quiescent standbystate to a small temperature increase at the ablation site (smallincrease in power applied), the output signal of the thermistor isessentially linear. By matching the output curve of the thermistor withthe generally linear response curve of the galvanic cell, two coincidentreference points can be determined. Referring to FIG. 4, there isillustrated a thermistor response curve and a galvanic cell responsecurve manipulated to be coincident from a point 0 to a point 1. Bycorrelating the temperature indication of the thermistor at these twopoints, with the current output (i_(s)) of the galvanic cell, thetemperature response can be linearly extrapolated to obtain atemperature reading correlated with the current output of the galvaniccell. That is, for any given current output of the galvanic cell, thetissue temperature of the ablation site can be determined. Thus, ifprobe 14 illustrated in FIGS. 1 and 2 is of the type shown in FIG. 3,calibration of the probe at the ablation site can be readily determined.Other methods for calibrating the current output with temperature canalso be employed, as set forth above.

Referring to FIG. 5, there is illustrated a block diagram of the majorcomponents necessary to control the power applied to a catheter probefor ablating an errant impulse pathway at an ablation site. FIG. 5 showsa temperature input circuit 90 for setting a reference voltageequivalent to the tissue temperature sought for an ablation site atwhich an ablation procedure is to be performed. The resulting outputsignal is transmitted through conductor 92 to a servo amplifier 94. Theservo amplifier provides an output signal on conductor 96 to control theoutput power of RF generator 98. A switch 100 controls operation of theRF generator. The RF energy output is impressed upon conductor 102. Ablocking capacitor 104 is representative of a high pass filter andblocks any DC component of the signal on conductor l02. Conductor 106interconnects the blocking capacitor with tip 30 of probe 14 andtransmits RF energy to the tip. Tip 30 irradiates ablation site 20 of anendocardium, wall, membrane, or other living tissue to be irradiatedwith RF energy. Tip 30 is of a substance, such as platinum or othermetal, having a first work function. Plate 34 displaced from tip 30, isof a substance, such as copper or other metal, having a second workfunction which is different from the first work function. Plate 34 is inelectrical contact with a mass of tissue 38 intermediate tip 30 and theplate. This tissue, being essentially a liquid and having electricalcharacteristics of a saline solution, serves in the manner of anelectrolyte interconnecting tip 30 and plate 34. The resulting galvaniccell formed, as discussed above, provides a DC output voltage v_(s)across conductors 106 and 108. Shunt impedance R1 heavily loads thegalvanic cell formed to convert the galvanic cell to a current source(i_(s)) to provide an output signal reflective of the tissue temperatureat ablation site 20. The output signal from the galvanic cell istransmitted through conductor 110 to a lowpass filter 112. The output ofthe lowpass filter is conveyed via conductor 114 to an operationalamplifier 120 of a calibration circuit 116. Additionally, a signalmeasurement and processing circuit 118, connected to conductor 102through conductor 103 to provide sampling of the output load voltage(V₀). It is also connected to conductor 107 through conductor 105 toprovide an input signal of the load output) current (I₀) sensed,processes the input signals to provide an indication of the impedance,power, and voltage and current levels. A readout 123, connected throughconductor 119 to signal measurement and processing circuit 118 provideseach of a plurality of indications of impedance, power, voltage level,current level, etc.

Variable resistors R3 and R4, in combination with operational amplifier120, are representative of adjustments to be made to correlate theoutput current (i_(s)) of the galvanic cell with the tissue temperatureof ablation site 20. Calibration circuit 116 can perform theabove-described correlation of the thermistor indicated temperature withthe current output signal of the galvanic cell to obtain a tissuetemperature indication of the ablation site as a function of the current(i_(s)) generated by the galvanic cell. A readout 122, connected viaconductors 124, 126 with the calibration circuit, may be employed toprovide an indication of the tissue temperature of the ablation site. Anoutput signal from the calibration circuit is also conveyed viaconductors 124 and 128 to servo amplifier 94. This output signal isreflective of the tissue temperature at the ablation site. Thereby, theservo amplifier receives an input signal reflective of the tissuetemperature at the ablation site. Circuitry of servo amplifier 94 willdetermine whether to raise or lower the tissue temperature of theablation site or to maintain it at its preset temperature. A commandsignal to increase, to decrease, or to maintain the power output of theRF generator is transmitted from servo amplifier 94 through conductor 96to the RF generator.

Referring to FIG. 6, there is illustrated a variant of probe 14 useablewith the present invention. The combination of first mapping a site ofinterest and then ablating the site is a lengthy procedure. Were itpossible to ablate a site identified during a mapping procedure withoutrelocating the probe or without replacing the mapping probe with anablating probe, significant time would be saved. FIG. 6 illustrates acatheter probe 130, which may be sufficiently flexible to position allor some of its length in contacting relationship with the surface of themyocardial tissue to be mapped. A tip 132, which may be similar to tip30 of probe 14, is disposed at the distal end. A plurality of mappingelectrodes, such as rings 134, 136, 138, 140 and 142 are disposedproximally along the probe from tip 132. These rings serve a function ofmapping the tissue of interest to identify and locate a site to beablated to destroy the circuit responsible for errant impulses. Forthese rings to work in the manner of tip 30, as described with referenceto FIGS. 1-5, the rings are preferably metallic and have a work functiondifferent from that of plate (or electrode) 34. One of a plurality ofconductors 144, 146, 148, 150, 152 and 154 interconnect the respectivetip and rings with the output of a switching circuit(s) 160. A dataacquisition circuit 162 is selectively interconnected through switchingcircuit 160 to each of rings 132-142 and possibly tip 132. The dataacquisition circuit collects data sensed by the rings and/or tips to mapthe tissue surface traversed by the probe. Upon detection of a site tobe ablated to destroy an impulse pathway (circuit), switch circuit 160switches to interconnect the respective ring (or tip) with RF generator164. Upon such interconnection, the respective ring (or tip) willirradiate the identified site with RF energy and the ablation function,as described above along with the tissue temperature control function,will be performed.

From this description, it is evident that upon detection of a sitelocated by performing a mapping function, ablation of the site can beperformed immediately without further movement or manipulation of thecatheter probe. Furthermore, the ablation function can be performed withthe circuitry illustrated in FIG. 5 to heat and maintain the tissue at apredetermined temperature until ablation is completed.

Empirically, it has been determined that the circuit and apparatus forablating tissue, as illustrated in FIG. 5, provides to a physician avery accurate indication of the tissue temperature at the ablation site.With such accuracy, ablation procedures are capable of being performedon thin wall tissue without fear of coagulation of the tip, adhesion oftissue to the tip or puncture, which fears exist with presently usedablation performing apparatus. Furthermore, accurate representation ofthe temperature at the ablation site is no longer critically dependentupon the orientation of the probe at the ablation site nor upon theextent of the depression of the tissue in response to the pressureexerted by the probe tip. Because of these very difficult to controlvariables, complete ablation of the errant impulse pathway was notalways achieved if the physician were overly cautious. Tip coagulation,sticking tissue and sometimes excessive injury to and puncture of thetissue occurred if the physician were more aggressive. These resultswere primarily due to the inaccuracy of the information conveyed to thephysician during the procedure and not so much due to poor technique.

As will become evident from the above description, tip 30 (and tip 132)does not need a thermistor or a thermocouple to set or determine thetemperature of the ablation site. Therefore, the probe can be smallerand more versatile than existing probes. Moreover, the probe can bemanufactured at a substantially reduced cost because it is more simplethan existing devices. Rings (or other electrodes) located on thecatheter can be used for mapping sites of errant impulses and any of therings (or other electrodes) can be used to irradiate the tissue at suchsite after identification of the site and without repositioning of thecatheter.

While the invention has been described with reference to severalparticular embodiments thereof, those skilled in the art will be able tomake the various modifications to the described embodiments of theinvention without departing from the true spirit and scope of theinvention. It is intended that all combinations of elements and stepswhich perform substantially the same function in substantially the sameway to achieve the same result are within the scope of the invention.

What is claimed is:
 1. Apparatus for mapping tissue to locate a site to be ablated and for ablating the site located, said apparatus comprising in combination: (a) a probe having a plurality of electrodes disposed therealong for placement adjacent the tissue to be mapped; (b) a data acquisition circuit for performing mapping functions in response to signals generated by the tissue proximate each electrode of said plurality of electrodes to identify and locate a tissue site to be ablated; (c) a temperature sensor for producing a signal reflective of the temperature sensed, said temperature sensor comprising a first electrode of said plurality of electrodes having a first work function, a second electrode having a second work function and an electrolyte in electrical contact with said first and second electrodes; (d) an RF generator for generating RF energy to ablate a located ablation site in response to the signal generated by said temperature sensor; (e) a switching circuit for selectively interconnecting said data acquisition circuit and said RF generator to each electrode of said plurality of electrodes; and (f) a plurality of conductors interconnecting said switching circuit with said plurality of electrodes.
 2. The apparatus of claim 1 wherein said plurality of electrodes includes a plurality of rings disposed in spaced apart relationship along said probe.
 3. The apparatus of claim 2 wherein said plurality of electrodes includes a tip electrode disposed at the distal end of said probe.
 4. The apparatus of claim 3 wherein said switching circuit interconnects only one ring of said plurality of rings and said tip electrode whichever corresponds with the site to be ablated.
 5. The apparatus of claim 1 wherein said apparatus includes means for serially performing the mapping and ablating functions.
 6. The apparatus of claim 5 wherein said switching circuit interconnects the electrode of said plurality of electrodes which corresponds with the site to be ablated.
 7. The apparatus of claim 1 wherein said switching circuit interconnects the electrode of said plurality of electrodes which corresponds with the site to be ablated.
 8. Apparatus for mapping tissue to locate a site to be ablated and for ablating the site located, said apparatus comprising in combination: (a) a probe having a plurality of electrodes disposed therealong for placement adjacent the tissue to be mapped; (b) a data acquisition circuit for performing mapping functions in response to signals generated by the tissue proximate each electrode of said plurality of electrodes to identify and locate a tissue site to be ablated; (c) a temperature sensor for producing a signal reflective of the temperature sensed, said temperature sensor comprising a first electrode of said plurality of electrodes having a first work function, a second electrode having a second work function and an electrolyte in electrical contact with said first and second electrodes; (d) an RF generator for generating RF energy to ablate a located ablation site in response to the signal generated by said temperature sensor; (e) a switching circuit for selectively interconnecting said data acquisition circuit and said RF generator to selected electrodes of said plurality of electrodes; and (f) a plurality of conductors interconnecting said switching circuit with the selected electrodes of said plurality of electrodes.
 9. The apparatus of claim 8 wherein said plurality of electrodes includes a plurality of rings disposed in spaced apart relationship along said probe.
 10. The apparatus of claim 9 wherein said plurality of electrodes includes a tip electrode disposed at the distal end of said probe.
 11. The apparatus of claim 10 wherein said switching circuit interconnects only one of the rings of said plurality of rings and said tip electrode whichever corresponds with the site to be ablated.
 12. The apparatus of claim 8 wherein said apparatus includes means for serially performing said mapping and ablating functions.
 13. The apparatus of claim 12 wherein said switching circuit interconnects only the selected electrodes of said plurality of electrodes which correspond with the site to be ablated.
 14. The apparatus of claim 8 wherein said switching circuit interconnects only the selected electrodes of said plurality of electrodes which correspond with the site to be ablated.
 15. Apparatus for mapping tissue to locate a site to be ablated and for ablating the site located, said apparatus comprising in combination: (a) a probe having a plurality of electrodes disposed therealong for placement adjacent the tissue to be mapped; (b) a data acquisition circuit for performing mapping functions in response to signals generated by the tissue proximate at least one selected electrode of said plurality of electrodes to identify and locate a tissue site to be ablated; (c) a temperature sensor for producing a signal reflective of the temperature sensed, said temperature sensor comprising a first electrode of said plurality of electrodes having a first work function, a second electrode having a second work function and an electrolyte in electrical contact with said first and second electrodes; (d) an RF generator for generating RF energy to ablate a located ablation site in response to the signal generated by said temperature sensor; (e) a switching circuit for selectively interconnecting said data acquisition circuit and said RF generator to at least one selected electrode of said plurality of electrodes; and (f) a plurality of conductors at least one conductor of said plurality of conductors interconnecting said switching circuit with each selected electrodes of said plurality of electrodes.
 16. The apparatus of claim 15 wherein said plurality of electrodes includes a plurality of rings disposed in spaced apart relationship along said probe.
 17. The apparatus of claim 16 wherein said plurality of electrodes includes a tip electrode disposed at the distal end of said probe.
 18. The apparatus of claim 17 wherein said switching circuit interconnects RF generator with one ring of said plurality of rings and said tip electrode.
 19. The apparatus of claim 15 wherein said apparatus includes means for serially performing said mapping and ablating functions.
 20. The apparatus of claim 19 wherein said switching circuit interconnects said RF generator with the electrode of said plurality of electrodes which corresponds with the site to be ablated.
 21. The apparatus of claim 15 wherein said switching circuit interconnects said RF generator with the electrode of said plurality of electrodes which corresponds with the site to be ablated. 